Geology Reference
In-Depth Information
8.13.5 Chemical upgrading ofREO
A chemical upgrading follows beneficiation to dissolve the RE compounds. Conven-
tional techniques for chemical dressing may include roasting, caustic soda fusion,
sulphuric acid attack at high temperatures and hydrochloric acid leaching. As bast-
naesite is a fluorocarbonate (REFCO
3
) chemical separation can be achieved either
by obtaining crystallised chloride hexhydrate RECl
3
6H
2
O or its hydroxide.
Subsequently, the bastnaesite concentrate is leached with hydrochloric acid at
10% to remove calcite, then dried and calcined. The process leads to a 90% REO
concentrate. A further digestion with HCl 30% yields a slurry with RECl
3
and
REF
3
in solid phase. Any fluorides present can be digested into soluble hydroxides
with caustic soda and recycled. Thereafter the whole slurry is treated with caustic
soda with the resulting cake re-chlorinated to convert hydroxides into chlorides. The
solution is then neutralised, filtered and evaporated to produce rare earth chlorides
(Gupta and Krishnamurthy, 1992). The acidic route of Bayan Obo first roasts the
REO concentrate for four hours with sulphuric acid in a rotary kiln at 400-500
o
C.
This process dissolves the minerals, producing sulphates RE
2
SO
4
, then removes any
fluorides and CO
2
. The mixture is then filtered and leached with water to obtain a
pure rare earth sulphate solution. Further treatment with ammonium bicarbonate
precipitates and allows a selective recovery of RE carbonates. The production of
RE chlorides is obtained by the treatment of carbonates with hydrochloric acid
(Bouorakima, 2011; Schüler et al., 2011).
Monazite concentrate, meanwhile, may be treated with caustic soda (60-70%) at
140
o
C to convert the REPO
4
into Na
2
PO
4
and RE hydroxides. Following filtra-
tion, the solution contains Na
2
PO
4
, which can be recovered as a byproduct after
having been subjected to evaporation and crystallisation. The resulting cake is
treated similarly to that of the bastnaesite process with HCl utilised to obtain RE
chlorides. However, as monazite contains Th and U, the HCl dissolution must be
critically maintained at 70-80
o
C and a pH 4-4.4 to precipitate the mildly radioac-
tive Th and U chlorides and convert them into valuable, albeit environmentally
questionable, byproducts. Consequently, protective measures are needed from the
extraction of the ore to the workings of slurry tailings and chemical processing.
Alternatively, monazite may be digested with sulphuric acid at 200-220
o
C to dis-
solve RE and Th sulphates and filtered out from spurious solids. Neutralisation
with ammonia and an addition of Na
2
SO
4
facilitates the precipitation of the dou-
ble sulphate RE
2
(SO
4
) Na
2
SO
4
nH
2
O. Additionally, REE can be precipitated
as oxalates from the sulphuric dissolution. In both cases the precipitate is then
essentially ready to follow the route of the aforementioned hydroxides (Gupta and
Krishnamurthy, 1992).
8.13.6 Isolation ofREElements
Separation and purification equipment falls under fine chemistry. Flexible and so-
phisticated apparatuses are used for the treatment of varied REO composition feeds
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